Volume 39, Issue 2 (March 2011)
Moisture Damage in Vented Air Space of Exterior Walls of Wooden Houses
Most of the exterior walls of wooden houses in Japan have a vented air layer between the exterior cladding and the insulation. This vented air layer is designed to dehumidify the exterior walls by discharging humidity to the outside and allowing outdoor air to enter, thereby decreasing the risk of condensation on the exterior wall during winter. It is assumed that the source of this moisture is the indoor air, and that the outdoor air is drier. However, the outdoor air is often highly humid during the rainy season and may become a source of moisture. The vented air layer also allows rain water to drain away. Any rain water penetrating through the exterior cladding is drained away through the vented air layer. However, rain can also enter the vented air space through the air inlets. Since it takes a certain amount of time for all the rain water to drain away, water may accumulate in the vented air layer and produce high humidity in the exterior wall. In order to evaluate the effectiveness of an exterior wall with a vented air layer, its hygrothermal characteristics should be investigated, taking the effect of rain water into consideration. This paper describes a case of moisture damage where stain appeared on the outer surface of the plywood wall in a wooden residential building. Experiments were carried out in climate chambers to clarify the conditions that are causing stain. Hygrothermal conditions in the vented air layer were simulated using heat, air, and moisture model, and the causes of staining were investigated.